For newly seeded areas, you want to use the mist setting of a hose nozzle,
and water until the surface just glistens. A heavier stream will wash seed
around. It'll fill in eventually, but it may be annoying to see the seed get
knocked around, after all your work.

A neighbor of mine found some sort of sprinkler that had a mist setting,
perfect for new grass seed. But, if there's any breeze at all, the mist just
blows away. Still, there were enough times when it was useful for an hour or
two at a time.

For very small areas where you repair the grass, you could lay burlap on top
of the seed, and weigh it down with baseball-size rocks. Water right through
the burlap. The grass will grow through it, and at that point, you can lift
it off without hurting the grass. Rinse the burlap, hang it up to dry, and
save it for the next time, like when snow piles damage small areas, or your
neighbor's stinking dog picks a spot and won't leave it alone.